Bone substitute for training and testing and method for making

ABSTRACT

A bone substitute that drills and cuts like bone for use in training and testing comprising an inner core of a foamable polymer or other soft material and an outer shell of a polymer such as an epoxy resin with a particulate filler such as aluminum oxide or silicon carbide added thereto together with, in some cases, titanium oxide to form a slurry for casting or molding around the inner core. Also provided is a method for making the bone substitute.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of prior filed application Ser. No. 08/825,192, filed Mar. 27, 1997 now U.S. Pat. No. 6,116,911.

STATEMENT OF GOVERNMENTAL INTEREST

This invention was made with Government support under Contract No. N00039-94-C-0001 awarded by the Department of the Navy. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The invention relates in general to substitutes for bone, and, in particular, to a bone substitute that has the look and feel, and cutting and drilling properties of human bone thereby making it especially useful as a bone model for teaching and training medical students and for testing surgical equipment.

Drilling bone to permit use of internal screws for fixation of fractures, to implant artificial joints, to fix intramedullary implants and to utilize various other procedures is a widespread and important surgical technique. Obviously, the above surgical procedures involve precise cuts and drilling of sensitive tissues.

Unfortunately, there is a shortage of human bone tissue on which to practice new techniques and procedures. Cadaver bone is difficult and often expensive to obtain and is a serious potential biohazard as well. Currently, surgeons practice new drilling techniques on blocks of plastic or polyurethane, assuming this material closely mimics the drilling behavior of live human bone which, however, is not the case.

Previous studies on the drilling of bone have focused on orthogonal cutting and machining, and wear of machine parts, but there is currently no easy way to comprehend data concerning distinguishing drilling behavior of materials for comparison. In any event, what is needed are new materials which when molded will drill and cut like bone in order to provide better training for medical students and more realistic testing for surgical equipment manufacturers.

SUMMARY OF THE INVENTION

The invention provides a bone substitute whose properties closely mimic real bone when drilled or cut and comprises an inner core comprising a foamable polymer or other soft material to mimic cancellous bone and an outer shell formed around the inner core to mimic compact bone. The outer shell comprises a polymer such as an epoxy resin and a particulate filler such as a mineral added thereto to form a slurry for casting or molding around the inner core.

The particulate filler, which hardens the bone substitute and reduces the amount of polymer required, includes, but is not limited to, hydroxyapatite, aluminum oxide (Al₂O₃), silicon carbide (SiC) or mullite. For even better results, titanium oxide (Tio) can be added along with either Al₂O₃ or SiC to modify the interaction between the polymer and the mineral and thereby reduce wear on surgical tools.

In one embodiment, the outer shell comprises an epoxy resin and from 5% to 15% by weight of Al₂O₃ and from 20-45% by weight of TiO. In another embodiment, the outer shell comprises the epoxy resin and from 2.5% to 30% by weight of SiC and from 20% to 45% by weight of TiO.

To make a bone of the invention the first step is to make a female mold from an original (human) bone part. Then a bone substitute part is created from the female mold and reduced by a uniform thickness. A mold is created from the bone substitute part to replicate an inner core of the bone substitute; the inner core is then molded from a foamable polymer and suspended in the female mold. Finally, the outer shell is formed by pouring or injecting the epoxy resin/Al₂O₃ (or SiC)/TiO slurry into the female mold with the inner core suspended therein.

The resulting bone substitute drills and cuts substantially like real bone thereby providing medical students with an accurate feel during surgical training, and equipment manufacturers with an accurate hardness for testing surgical devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-section of the bone substitute of the invention.

FIG. 2 is a bar graph illustrating drilling data from dog, lamb and cow bones and various compositions of bone substitutes of the invention.

FIG. 3 is a chart showing load as a function of particle diameter for different substances added to an epoxy resin comprising the outer shell of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a bone substitute that has the look and feel, and cutting and drilling properties that closely mimic real bone. As shown in FIG. 1, the invention comprises an inner core 10 made of a foamable polymer or other soft material and an outer shell 12 comprising a polymer and a particulate filler.

The filler is to increase the hardness, toughness and/or resistance of the polymer to drilling and cutting. The polymer and particulate filler form a slurry for casting around the inner core.

In laboratory tests of the invention, discussed below, two families of epoxy resins were chosen for the polymer for ease of processing: Bisphenol A and Bisphenol F. However, other thermosetting as well as thermoplastic polymers such as those listed in Table 1 below can also be used in the invention.

TABLE 1 ABS Polyalkalene Ether ABS/PA Polyallomer ABS/PC Polyamide (Nylon) ABS/PVC Polybutadiene Acetal Polybutylene Acrylic Polycarbonate Acrylonitrile Copolymer Polyester (Saturated) Alkyd Polyester (Unsaturated) Allylic Esters or Allyls (DAP,DAIP) Polyether, chlorinated ASA (Acrylic-styrene-acrylonitrile) Polyethylene Bis-maleimides Polyimide (Polyamide-imide) Cellulosics Polyphenylene Sulfide Cyanate/Cyanamide Polypropylene Epoxy Polystyrene Ethylene Vinyl Acetate Polysulfone Fluorocarbon Polymers : Polyurethane Fluorinated Ethylene-Propylene Polyvinyl Acetate (FEP) Perfluoroalkoxy (PFA) Polyvinyl Chloride Polychlorotrifluoroethylene (CTFE) Polyvinylidene Chloride Polytetrafluoroethylene (TFE) Polyxylylene Polyvinylfluoride (PVF) Silicone Polyvinylidene Fluoride (PVDF) Styrene-acrylonitrile (SAN) Furan Styrene-maleic-anhydride (SMA) Ionomer Urea-Formaldehyde Melamine-Formaldehyde Vinyl Ester Phenolic

The particulate filler comprises a mineral, which, as noted above, hardens the bone substitute and reduces the amount of polymer required. Suitable minerals include, but are not limited to, hydroxyapatite, aluminum oxide (Al₂O₃), silicon carbide (SiC) and mullite.

In the laboratory tests, as discussed below, even better results were obtained by adding a second filler, titanium oxide (TiO), to either Al₂O₃ or SiC to modify the interaction between the epoxy resin and the mineral and thereby reduce wear on surgical tools.

In one embodiment, the outer shell comprises an epoxy resin and from 5% to 15% by weight of Al₂O₃ and from 20-45% by weight of TiO. In another embodiment, the outer shell comprises the epoxy resin and from 2.5% to 30% by weight of SiC and from 20% to 45% by weight of Tio. Best results were obtained with particle size for the Al₂O₃ being 100 microns or less and for the SiC being 10 microns or less.

To fabricate a bone substitute of the invention the first step is to make a female mold from an original (human) bone part. Then a bone substitute part is created from the female mold and reduced by a uniform thickness. A mold is created from the bone substitute part to replicate an inner core of the bone substitute; the inner core is then molded from a foamable polymer and suspended in the female mold. Finally, the outer shell is formed by pouring or injecting, for example, the epoxy resin/Al₂O₃ (or SiC)/TiO slurry into the female mold with the inner core suspended therein.

As shown in Table 2, different composite samples were formed in laboratory tests, consisting of three different epoxy resin systems, EPON® Resins 815, 826 and 862 (EPON is a registered trademark of Shell Chemical Company).

TABLE 2 Composition Table Resin Filler A Filler B Sample No. Test Date (Drilling) 826/V-40 None None 7 19-Nov-96 815/V-40 None None 8 19-Nov-96 826/V-40 35% 325 Mullite None 11 826/V-40 56.6% 325 Mullite None 12 815/V-40 40% 325 Mullite None 13 815/V-40 40% 325 Mullite None 13 21-Nov-96 815/V-40 60.8% 325 Mullite None 14 815/V-40 60.8% 325 Mullite None 14 21-Nov-96 862/3274 51.7% 100 Mullite None 15 862/3274 51.7% 100 Mullite None 15 21-Nov-96 862/3274 64.6% 100 Mullite None 16 862/3274 64.6% 100 Mullite None 16 21-Nov-96 815/V-40 47% 100 Mullite None 17 815/V-40 47% 100 Mullite None 17 21-Nov-96 815/V-40 62% 100 Mullite None 18 815/V-40 62% 100 Mullite None 18 21-Nov-96 826/V-40 24% 100 Mullite None 19 826/V-40 62% 100 Mullite None 19 21-Nov-96 826/V-40 49% 100 Mullite None 20 826/V-40 49% 100 Mullite None 20 21-Nov-96 826/V-40 34.2% T64-60 None 21 826/V-40 34.2% T64-60 None 21 21-Nov-96 826/V-40 52% T64-60 None 22 826/V-40 52% T64-60 None 22 21-Nov-96 815/V-40 61% T64-60 None 23 815/V-40 61% T64-60 None 23 21-Nov-96 815/V-40 38.7% T64-60 None 24 815/V-40 38.7% T64-60 None 24 21-Nov-96 862/3274 71.7% T64-60 None 25 862/3274 71.7% T64-60 None 25 21-Nov-96 862/3274 52% T64-60 None 26 22-Nov-96 826/V-40 57% T64-200 None 27 22-Nov-96 826/V-40 42% T64-200 None 28 22-Nov-96 815/V-40 43.3% T64-200 None 29 22-Nov-96 862/3274 55% T64-200 None 30 22-Nov-96 862/3274 65% T64-200 None 31 22-Nov-96 862/3274 71% AC99-325 Ll None 32 22-Nov-96 862/3274 60.6% AC99-325 Ll None 33 22-Nov-96 815/V-40 50% AC99-325 Ll None 34 22-Nov-96 815/V-40 61% AC99-325 Ll None 35 22-Nov-96 826/V-40 24.7% AC99-325 Ll None 36 22-Nov-96 826/V-40 48% AC99-325 Ll None 37 22-Nov-96 862/3274 52.3% A10 ung (?) None 38 22-Nov-96 862/3274 40.4% A10 ung (?) None 39 22-Nov-96 815/V-40 30% A, 10 μm None 40 3-Dec-96 815/V-40 15% A, 10 μm None 41 3-Dec-96 826/V-40 39% A, 10 μm None 42 3-Dec-96 826/V-40 10.75% A, 10 μm None 43 3-Dec-96 826/V-40 29% Premalox None 44 3-Dec-96 826/V-40 46% Premalox None 45 3-Dec-96 815/V-40 50% Premalox None 46 3-Dec-96 815/V-40 39.8% Premalox None 47 3-Dec-96 862/3274 31.6% Premalox None 48 3-Dec-96 862/3274 55.5% Premalox None 49 3-Dec-96 862/3274 10.24% Q-Cel 2116 None 50 3-Dec-96 862/3274 5.43% Q-Cel 2116 None 51 3-Dec-96 862/3274 42.3% TiO₂ None 52 3-Dec-96 862/3274 56% TiO₂ None 53 3-Dec-96 862/3274 25.5% TiO₂ None 54 3-Dec-96 826/V-40 58.2% AC99-100 None 55 3-Dec-96 815/V-40 33.5% 3μ SiC None 56 3-Dec-96 Sawbones None None 58 5-Dec-96 815/V-40 54% 3μ SiC None 59 6-Dec-96 826/V-40 5.6% 3μ SiC None 60 6-Dec-96 826/V-40 29% 3μ SiC None 61 6-Dec-96 862/3274 32% 3μ SiC None 62 6-Dec-96 862/3274 46.5% 3μ SiC None 63 19-Nov-96 826/V-40 27.5% AC99-100 None 64 6-Dec-96 862/3274 59.85% AC99-100 None 65 6-Dec-96 862/3274 69.5% AC99-100 None 66 6-Dec-96 815/V-40 48% AC99-100 None 67 6-Dec-96 815/V-40 64% AC99-100 None 68 6-Dec-96 826/V-40 16.8% 20μ SiC None 69 6-Dec-96 826/V-40 44.8% 20μ SIC None 70 6-Dec-96 826/V-40 26.4% TiO₂ 5.6% α Al₂O₃, 0.3μ 71 6-Dec-96 815/V-40 56% 20μ SiC None 72 6-Dec-96 815/V-40 33% 20μ SiC None 73 6-Dec-96 862/3274 55.6% 20μ SiC None 75 6-Dec-96 862/3274 67.3% 20μ SiC None 76 6-Dec-96 862/3274 53.3% 100μ SiC None 77 6-Dec-96 862/3274 Neat Resin None 78 9-Dec-96 862/3274 48.68% TiO₂ 7.5% α Al₂O₃ 79 9-Dec-96 862/3274 45.71% TiO₂ None 80 10-Dec-96 862/3274 48.63% TiO₂ 8.17% 3μ SiC 81 11-Dec-96 862/3274 45.24% TiO₂ 14.57% 3μ SiC 82 11-Dec-96 862/3274 48.63% TiO₂ 8.17% 3μ SiC 83 12-Dec-96 862/3274 25.23% 3μ SiC 23.51% TiO₂ 84 13-Dec-96 862/3274 31.2% TiO₂ 9.92% Premalox 85 13-Dec-96 862/3274 41.54% TiO₂ 5.73% 3μ SiC 86 16-Dec-96 862/3274 42.44% TiO₂ 3.82% 3μ SiC 87 16-Dec-96 862/3274 30.72% TiO₂ 9.18% Premalox 88 16-Dec-96

Except in two cases, each sample was filled with a variety of particulate minerals and titanium oxide (together fillers) of different diameters, and different volume concentrations as shown in Table 3. Samples made were 1″×6″×⅛″ in size, and were allowed to cure for a minimum of two days before any experiments were performed.

TABLE 3 Filler Diameter % Resin Curing Agent Treatment A-10 Ung 100 μm 15.03 EPON 815 EPON V-40 Cure Rm. Temp. overnight Alumina Oxide 29.9 10.75 EPON 826 EPON V-40 Blow Dry, Cure Rm. Temp. 38.98 40.39 EPON 862 EPICURE 3274 52.33 Premalox 10 SG 0.25 μm 28.64 EPON 826 EPON V-40 Cure Rm. Temp. overnight Alumina Oxide 45.87 39.83 EPON 815 EPON V-40 Cure Rm. Temp. overnight 50.65 31.58 EPON 862 EPICURE 3274 Blow Dry, Cure 150° F. 2 hrs 55.54 Mullite ˜149 μm 24.02 EPON 826 EPON V-40 Blow Dry, Cure 150° F. 1 hr 100 Mesh 48.97 46.98 EPON 815 EPON V-40 Blow Dry, Cure Rm. Temp. overnight 62.4 51.66 862 EPICURE 3274 Blow Dry, Cure Rm. Temp. overnight 64.62 Mullite ˜44 μm 34.91 EPON 826 EPON V-40 Blow Dry, cure Rm. Temp. overnight 325 Mesh 56.62 39.69 EPON 815 EPON V-40 Blow Dry, Cure Rm. Temp. overnight 60.8 41.96 EPON 862 EPICURE 3274 Blow Dry, Cure rm. Temp. overnight 62.93 Aluchem AC99-100 ˜149 μm 27.5 EPON 826 EPON V-40 Blow Dry, Cure 150° F. 2 hrs Tabular Alumina 58.28 48.19 EPON 815 EPON V-40 Cure Rm. Temp. overnight 64.1 59.85 EPON 862 EPICURE 3274 Blow Dry, Cure Rm. Temp overnight 69.46 AC99-325 Ll ˜44 μm 24.71 EPON 826 EPON V-40 Blow Dry, Cure Rm. Temp overnight Tab. Alumina Ground 47.99 Low Iron 50.18 EPON 815 EPON V-40 Blow Dry, Cure 15° F. 3 um 61.58 60.61 EPON 862 EPICURE 3274 Blow Dry, Cure Rm. Temp. overnight 71.15 T64-60 100 μm 34.24 EPON 826 EPON V-40 Blow Dry, Cure 150° F. 2.5 hrs. Tabular Alumina 52.05 38.68 EPON 815 EPON V-40 Blow Dry Cure rm. Temp overnight 61.19 52.05 EPON 862 EPICURE 3274 Blow Dry, Cure 150° F. 3 hrs 71.77 Silicon Carbide 3 μm 5.64 EPON 826 EPON V-40 Cure Rm. Temp. overnight 29.05 31.9 EPON 862 EPI-CURE 3274 Blow Dry, Cure 150° F. 2 hrs 46.51 33.52 EPON 815 EPON V-40 Blow Dry, Cure Rm. Temp. overnight 54.1 Silicon Carbide 100 μm 44.06 EPON 882 EPI-CURE 3274 Blow Dry, Cure 150° F. 3 hrs 62.35 38.24 EPON 815 EPON V-40 Blow Dry, cure 150° F. 2 hrs. 53.33 20 μm 32.74 EPON 815 EPON V-40 Blow Dry, Cure 150° F. 3.5 hrs. 56.29 16.79 EPON 826 EPON V-40 Blow Dry, Cure Rm. Temp. overnight 44.8 55.6 EPON 862 EPI-CURE 3274 Blow Dry, Cure Rm. Temp. overnight 67.31 Titanium (IV) Oxide 25.55 EPON 862 EPI-CURE 3274 Blow Dry, Cure rm. Temp. overnight 43.54 42.3 Cure Rm. Temp. overnight 56.2 Same 47.57 EPON 862 EPICURE 3274 Cure Rm. Temp. overnight (Dried 150° F. 2 hrs) 65.71 25.77 EPON 826 EPON V-40 Cure Rm. Temp. overnight 61.29 56.8 EPON 815 EPON V-40 Cure Rm. Temp. overnight 71.17 Q-Cell 2116 5.43 EPON 862 EPICURE 3274 Blow Dry, Cure Rm. Temp overnight 10.24 T64-200 20 μm 55.12 EPON 862 EPICURE 3274 Blow Dry, Cure Rm. Temp. overnight Tabular Alumina 64.88 43.32 EPON 815 EPON V-40 Blow Dry, Cure Rm. Temp. overnight 67.85 T64-200 41.89 EPON 826 EPON V-40 Blow Dry, Cure Rm. Temp. overnight Tabular Alumina 57

Bone specimens tested for comparison were bovine, lamb and dog, the bovine and lamb obtained from a local store, kept cold, and used within one week. The length of time the canine bone had been frozen was unknown. Bovine bone tissue has previously been shown to be similar to human bone tissue with respect to many physical and structural properties. Also tested was a bone substitute product manufactured by Pacific Research Laboratories, Inc., called Sawbones®.

A standard surgical drill, the Maxidriver, was obtained and used for all drilling tests. A standard ⅛″ twist drill bit was used, a new bit for each sample. The drill was driven by nitrogen gas and all tests were performed at 110 psi, which results in a speed of approximately 900 rpm. The drill was clamped to the bottom of an INSTRON tensile machine, and samples were attached to a load cell and lowered onto the rotating drill.

Data was recorded for a given feed rate (usually two in/min) and fed to a personal computer. Information retrieved was load versus percent extension. A minimum of six holes were drilled in each sample with the same drill bit. A new drill bit was used for each bone or composite sample.

FIG. 2 provides comparative results between the dog, lamb and cow samples, the Sawbones® sample and ten samples of the invention (the sample numbers refer to the numbers in Table 2). FIG. 3 with its accompanying legend illustrates in chart form the performance of each sample from Table 2 as a function of filler particle diameter. The area where real bone falls is indicated by the hash marked area between approximately 7.5 and 12.5 on the load or y-axis. The legend can be used in conjunction with Table 4 to determine the performance of each sample.

TABLE 4 862/3274 % 826/V-40 % 815/V-40 % 826/3234 % TiO₂ A1 42.3 B1 45.5 C1 54.71 A2 56 A3 25.5 Premalox F1 31.6 D1 29 E1 50 F2 55.5 D2 46 E2 39.8 SiC-3 um I1 32 H1 29 G1 33.5 I2 46.5 H2 32 G2 54 SiC-20 um P1 55.6 N1 16.8 O1 56 P2 67.3 N2 44.8 O2 33 T64-200 M1 55 K1 57 L1 43.3 M2 65 K2 42 Mullite-325 Q1 63 R1 35 S1 40 Q2 42 R2 56.5 S2 60.8 AC99-325 Ll T1 71 V1 24.7 U1 50 T2 60.6 V2 48 U2 61 Mullite-100 W1 51.7 Y1 24 X1 47 W2 64.6 Y2 49 X2 62 T64-60 BB1 71.7 Z1 34.2 AA1 61 BB2 52 Z2 52 AA2 38.7 A10-Ung CC1 52.3 EE1 39 DD1 30 CC2 40.4 EE2 10.8 DD2 15 SIC-100 um FF1 53.3 AC99-100 HH1 59.85 GG1 58.2 II1 48 HH2 69.5 GG2 27.5 II2 64 Sand KK1 66 LL1 61.3 JJ1 70 KK2 47.5 LL2 25.8 JJ2 56.8

Finally, Table 5 excerpts the best performing combination of epoxy resin, mineral and, in some cases TiO, for forming the outer shell of the substitute bone of the invention.

TABLE 5 Filler 2 Table 2 Resin Filler 1 (weight Sample (resin/hardner) (weight %) (test data) %) (test data) Nos. Shell Epon 815/V40 35%-60% 39%-50% 0.25 um Al₂O₃ none 46, 47 100/44 pbw 30%-35% 33.5% 3.0 um SiC none 56 60%-65% 61% 100 um T64-60 tabular Al₂O₃ none 23 25%-35% 30% 10 um A-10 Al₂O₃ none 40 Shell Epon 826/V40 35%-60% 35%-56.6% 44 um mullite none 11, 12 100/100 pbw 20%-35% 24% 100 um mullite none 19 20%-55% 24.7%-48% 44 um AC99-325 none 36, 37 ground tabular Al₂O₃ 25%-35% 29% 0.25 um Al₂O₃ none 44 10%-50% 16.8%-44.8% 20 um SiC none 69, 70 Shell Epon 862/3274 35%-55% 40.4%-52.3% 10 um A-10 Al₂O₃ none 38, 39 100/44 pbw 25%-40% 31.6% 0.25 um Al₂O₃ none 48 30%-50% 32%-46.5% 3 um SiC none 62, 63  5%-15% 9.2%-9.9% 0.25 um Al₂O₃ 45%-20% 31.2%-30.7% TiO₂ 85, 88 2.5%-30%  3.8%-25.2% 3 um SiC 45%-20% 42.4%-23.5% TiO₂ 84, 86, 87

The resulting bone substitute of the invention drills and cuts substantially like real bone thereby providing medical students with an accurate feel during surgical training, and equipment manufacturers with an accurate hardness for testing surgical devices. 

We claim:
 1. A bone substitute useful as a bone model for teaching and training students and for testing surgical equipment, the bone substitute comprising: an inner core comprising a foamable polymer; and an outer shell formed around the inner core, the outer shell comprising an epoxy resin and silicon carbide (SiC) added to the epoxy resin.
 2. A bone substitute useful as a bone model for teaching and training students and for testing surgical equipment, the bone substitute comprising: an inner core comprising a foamable polymer; and an outer shell formed around the inner core, the outer shell comprising a polymer, and silicon carbide (SiC) and titanium oxide (TiO) added to the polymer to form a slurry for casting or molding around the inner core, said bone substitute exhibiting properties substantially like real bone useful for training and testing.
 3. A bone substitute comprising: an inner core comprising a foamable polymer; and an outer shell formed around the inner core, the outer shell comprising an epoxy resin and from 2.5% to 30% by weight of silicon carbide (SiC) and from 20% to 45% by weight of titanium oxide (TiO) added to the epoxy resin to form a slurry for casting or molding around the inner core, said bone substitute exhibiting properties substantially like real bone useful for training and testing. 